1. Field of the Invention
The invention relates to the field of analytical measuring technology and serves for detecting gaseous substances by absorbing preferably of IR-laser radiation. It can be used for detecting harmful substances in the atmosphere, for the process control of chemical phase reactions and for analyzing burn-up products, for example in laser surgery.
2. Description of the Related Art
An overview of known processes for detecting gaseous substances as well as a discussion of the advantages of spectroscopic and specifically laser spectroscopic processes can be found, for example, in P. L. Meyer and M. W. Sigrist Rev. Sci. Instrum. 61(7), July 1990, p. 1779 (named MS in the following). For detection, spectroscopic processes use the different absorption behaviour of the molecules, which, for example, results from the structure of the oscillation-rotation-spectrum in the infra-red spectral region. The weakening of the optical signal on a certain wave length is registered after passage through a gaseous medium, or the light energy absorbed in the medium, for example by means of an opto-ocoustic-detector (MS). In addition to absorption on a wavelength which is characteristic for the substance to be detected, one uses the differential absorption. In this case, the medium to be examined is irradiated by light at several and mostly two wavelengths either simultaneously or in following short sequence and the different absorption behaviour for the different wavelengths is used for the analysis. The DIAL-method is known, the differential absorption with LIDAR, in which two laser pulses of two different wavelengths are transmitted simultaneously or following in short sequence, one of the wavelengths being in resonance with the substance to be detected while the other wave length is not absorbed by this substance. The signals of both wavelengths scattered back by the aerosols in the air and then received again are differentiated and in this way, the concentration of the substance to be detected and the location at which this substance is located is localized via the travel time of the light pulse.
For detection in the infra-red spectral region, molecular gas lasers, specifically CO.sub.2 are used as the light transmitting source, and for the various wavelengths, laser lines of different oscilation-rotation transitions of the lasing molcules are used.
However, the absorption lines of different substances in the infrared region often lie close to one another so that although the absorption on a laser line is different, it is determined by different substances, which greatly limits the selectivity of this method. Moreover, although the absorption spectra are structured, they are wide. If a different substance to that which is to be detected only has a low net absorption but a considerably greater concentration on the resonance line of the substance to be detected, both substances cannot be differenciated from one another by means of the absorption. On account of the absorption of water vapor with this method, these interferences have the effect that many substances in the atmosphere cannot be detected and that for many other substances, the minimal detectable concentration in the atmosphere is greatly limited (MS).